Mr. McConnell has told colleagues he expects the House impeach Mr. Trump quickly, possibly by Thanksgiving, an educated hunch based on the pace of the inquiry so far and Speaker Nancy Pelosi’s decision to keep the inquiry narrowly focused on Mr. Trump’s dealings with Ukraine. He plans to move swiftly too, he told colleagues, using the approach of Christmas to force the Senate to complete its work before the beginning of 2020.
Yet an impeachment trial is a spectacle that is by its nature unpredictable, and most of the senators who will act as jurors were not around for the last one, of Bill Clinton in 1999. Mr. McConnell reminded senators that Chief Justice John G. Roberts Jr. would preside over the trial, and would have wide latitude in handling motions that might be made, including any motion to dismiss the charges that Republicans might try to put forward to short circuit the process.
Mr. McConnell’s declaration that the Senate would move forward was in part designed to show he had no choice, an effort to deflect criticism from conservatives outraged that the Senate would even consider impeachment.
On Wednesday, Senator Lindsey Graham, Republican of South Carolina, pushed for Senate Republicans to write a letter to Ms. Pelosi declaring that they would not remove the president. But some senators raised objections, worrying that some of their colleagues would not want to sign on, a result that would expose disunity among Republicans. Mr. Graham’s colleagues said they believe they staved off the letter, which they viewed as a mistake.
Mr. McConnell has made it clear that he plans to sit down with Senator Chuck Schumer of New York, the Democratic leader, to see if they can find a mutually acceptable way to move forward as Democrats and Republicans did in 1999 when they unanimously agreed on the framework for the impeachment trial. The Senate is much more polarized now, though Mr. Schumer this week held out hope.
“We have to do this trial in a fair and bipartisan way and I hope that Leader McConnell would obey those strictures,” Mr. Schumer said. In the battle for Senate control, Democrats have their own political risks to consider since impeachment could prompt a backlash against some of their candidates if enough voters conclude that the president was pursued unfairly.
Just 15 senators remain in office from the time Mr. Clinton was put on trial. Mr. McConnell warned them of the weight of the trial, where they can be required to be on the floor all afternoon six days a week without speaking — a major challenge for senators who relish their chances to be heard.
“It will mean day after day sitting in chamber, listening to the two sides, writing questions for them to answer that go through the chief justice,” said Ms. Collins, one of the Republicans who voted to acquit Mr. Clinton 20 years ago. “Members who have not been through this before will find it is a great deal of work.”